Humans and many animals have binocular vision provided by two eyes which look in the same direction. Two parallel aligned but spaced eyes deliver two slightly different images of the same scene. This is due to the 4 to 6 cm separation between the eyes, which makes each eye have a slightly different viewpoint. The images from these two different viewpoints are sent to the brain, and this difference, which is termed parallax, is interpreted as depth. The nearer an object is to the eyes, the greater the difference between the two images. From the difference between the two images, the brain can display an impression of depth.
Stereoscopic image displays, such as stereoscopic projection for example, are based on the same principle: two slightly different images, one for the left eye and one for the right eye, are displayed by some means, e.g. projected onto a screen. A corresponding image modulating system enables the left eye to see only the left eye image, and the right eye to see only the right eye image.
There are at least three types of electronic stereo-3D-devices:                a) devices which produce two different images which are so arranged that the left and right eye can only see the relevant image, such as Helmet Mounted Devices (HMD) or virtual reality (VR) headsets,        b) devices which project a single virtual image at a distance which is viewed by both eyes in a normal way, such as Head-Up Displays (HUD), and        c) viewing glasses which influence the way each eye sees the relevant image on the display. These glasses may have liquid crystal-shutter glasses or polarisers or include colour filters such as red and green filters.        
HMDs are basically wearable monitors. To allow stereoscopic vision, an image is projected onto a beam splitter in front of each eye. For VR headsets a miniature liquid crystal display (LCD) can be placed in front of each eye.
In active stereo, shutter glasses or shutter screens are used in conjunction with a normal CRT, digital light processing (DLP), or equivalent monitor or projector. The two images required for stereoscopic vision are displayed in turn. For a fraction of a second the image dedicated to the left eye is displayed, after that the image for the right eye appears for the same duration of time, and so on. The job of the glasses is to prevent the left eye from seeing the image dedicated to the right eye and vice versa. To do this, light is blocked by a shutter. The frequency of the shutter is adapted to the frequency of the displayed images. The blocking can be done by having a filter which alternates between opaque and transparent.
Passive stereo displays are based on the principle of different light polarisations or of differences in frequency (colour). For example, the viewer wears a pair of glasses containing two oppositely polarised lenses or filters, one for the left eye and one for the right eye. The light from each of two projected images is polarised differently and can pass only through its corresponding filter. If the images are provided by means of a single projector, the projector alternates the left eye information with the right eye information at double refresh rate. A screen in front of the projector's lenses alternates the polarisation of the projected image in such a way that the image of each eye passes only through its corresponding polarising filter of the pair of passive stereo glasses. If the images are provided by means of two projectors, one projector displays the left eye information and the other display the right eye information, both at a standard refresh rate. A polarising filter mounted in the optical path of each projector ensures that the correct information passes through its corresponding filter in the pair of passive stereo glasses. If the projectors are LCD projectors, they may be internally polarised, thus eliminating the requirement for external filters. The two-projector approach has the added value over the one-projector approach of providing higher brightness.
Stereoscopic image display may be used, a.o. in keyhole surgery, in entertainment applications, such as gaming environments, in training environments such as in flight simulators, ship bridge simulators, air traffic control, mission rehearsal simulators, and in visualisation of large data-sets, such as in car design for example.
Currently, projectors can display useful active stereo 3D-images from an image source such as an image generator (IG) if the vertical frequency is typically greater than 96 Hz. Typical large venue systems may use three projectors to produce a large image by tiling. Such a prior art set-up for active stereo 3D projection is shown in FIG. 1. It comprises three projectors 2, 4, 6 which each are connected to an image source 8, from which the projectors 2, 4, 6 receive images to be displayed on a display screen 10. An inverter 12 may be provided which is coupled both to the image source 8 and to an IR-emitter (infrared emitter) 14 for sending signals to shutter glasses (not represented), so as to be in phase with the displayed images, i.e. so that the left eye only receives images intended for the left eye, and so that the right eye only receives images intended for the right eye. The presence of an inverter 12 is technology dependent: when using projectors designed such that there is a delay of one image frame between reception and projection of images, and therefore an inverter 12 is necessary to make the shutter glasses to be in phase with the displayed images, but for other types of projectors such inverter may not be necessary.
According to an alternative prior art embodiment, as represented in FIG. 2, which e.g. uses Barco Galaxy projectors, obtainable from Barco Projection Systems, Kuurne, Belgium, the inverter for the IR signals may be integrated into one of the projectors, so as to add an adjustable delay to the shuttering to overcome the processing delay of the image in the projectors.
If a vertical frequency of 96 Hz is used for displaying the stereoscopic image, then the left and right images are updated at only 48 Hz. This results in flickering of the 3D-image, which is annoying and fatiguing, and is preferably to be avoided.
Furthermore IGs generating images for both eyes each at 48 Hz or more have to be powerful, and thus are expensive. It would be much cheaper if images could be generated at a lower frequency. However, displaying these images at these lower frequencies would certainly result in flickering images, and is thus not desirable.
On the other hand, if the 3D-image is displayed at a different vertical frequency greater than the input frequency in order to reduce flicker, the projectors will have asynchronously up converted the frequency. In this case, there is no longer a frequency nor phase relation between a displayed image and the IG-image. Therefore, in a multiple projector system, the stereo performance will be poor as each projector creates its own refresh rate that is not synchronised with the others and, for active stereo systems, with the control signal of the shutter glasses. Implementing the emitter phasing logic as in the set-up of FIG. 2 cannot overcome the differences between these images and the emitter control signal. Even if the control signal for the shutter glasses was generated by a projector then the glasses would only be synchronised with one projector channel.